High tension electrical inductive apparatus



United States Patent 3,538,470 HIGH TENSION ELECTRICAL INDUCTIVEAPPARATUS Bernard Crugnola, Tresserve, France, assignor to AlsthomSavoisienne, Saint Ouen, France, a corporation of France Filed Mar. 11,1969, Ser. No. 806,075 Claims priority, application France, Mar. 12,1968, 2 805 Int. (:1. limit 15/04, 15/14 US. Cl. 33669 6 Claims ABSTRACTOF THE DISCLOSURE The present invention relates to high tensionelectrical inductive apparatus, and more particularly to extra hightension transformers and improvements in the effectiveness of insulationbetween the transformer windings and transformer housing.

It is known to provide sufficient space between metallic housings, andthe windings of electrical apparatus to provide a potential gradient ofthe electrical field which is compatible with the insulatingeffectiveness of the insulation material in the space, and further toprovide for sufiicient separation so that the stray magnetic field doesnot cause excessive stray losses. In concentric windings, equalpotential lines are not perpendicular to the axis of the concentricwindings at the ends thereof; in other words, the equipotential lineswill not be parallel to the metallic masses at the ends of the axis butrather will present a different distribution.

It is an object ofthe present invention to provide a high tensionelectrical apparatus in which the efiectiveness of insulating materialsis improved.

SUBJECT MATTER OF THE PRESENT INVENTION Briefly, concentric windings aresurrounded, at the outside, by cylinders formed of metallic armatures soarranged as to form an electrostatic image of the windings themselves,the image being arranged and located with respect to an imaginarycylinder having an axis congruent with the axis of the concentricwinding, and a diameter slightly larger than the external diameter ofthe largest winding. The term electrostatic image as used in thisspecification may be defined as an arrangement of location of suchmetallic armatures, or metal sheets such that the capacity between thearmatures will, itself, cause a potential gradient distribution in thatspace outside of the winding, which is symmetrical, and a mirror imageof the potential gradient distribution between the layers of the windingitself.

The invention will be described by way of example with reference to theaccompanying drawings, wherein:

FIG. 1 is a cross sectional view through half of a toroidal-typewinding, arranged in accordance with the prior art, and illustratingequally potential lines;

FIG. 2 is a view similar to FIG. 1, in which metallic armatures areplaced in accordance with the present invention, also showing theresulting equipotential lines.

The basis for the present invention can best be understood by referringto FIG. 1, in which a cross-section, in highly schematic form, along aradial plane of the outer end of an electrical apparatus shown. A groupof concentric layers of windings, C C C only half of which are shown,must be considered to be wound in a toroid in a plane perpendicular tothe figure. A metal shield, for example, terminating in a rounded loop Awill be substantially at the potential of the outside turn of layer CThe distribution of equal potential lines will then be as shown by thedashed lines in FIG. 1. This distribution is determined by surroundingmetallic masses, in which the magnetic core is indicated by M the outerhousing by M and a metallic mass which may be an adjacent column, or avertical wall of the apparatus itself indicated by M M is assumed to begrounded.

The equipotential lines, shown in broken form, of the electrical fieldwhich will arise during operation clearly show that they are not at allinstances parallel to the mass M but rather strongly inclined withrespect thereto. Insulating material, and left off from the drawing forthe purpose of clarity, located between the windings and the housingwall M will thus be traversed by an electrical gradient which will havehorizontal and vertical components (with respect to FIG. 1). Suchinsulating material is thus electrically unevenly loaded. Insulatingmaterial frequently has its highest insulating effectiveness, that isresistance to breakdown, transverse to its major surface. Thus, mosteffective resistance to flashover is transverse to the surface MInclination of the electrical field will provide a vector componentwhich is not trans verse, but parallel to surface M FIG. 2 illustratesthe solution to the problem, the windings C to C being identical tothose shown in FIG. 1, and the internal magnetic circuit M as well asthe housing or otherwise shown metallic outer part M M also beingidentical. In accordance with the invention, metallic equipotentialarmatures A A A are located in the housing to surround the windinglayers C C C from the outside, and so disposed that they will be anelectrostatic image of the windings. The image is taken with respect toan imaginary cylinder, formed by the layers C C C the axis of thecylinder being congruent with the axis of the windings. The cylinderitself which, of course, is imaginary is indicated by the broken line Bin FIG. 2. It will be seen that the windings have axial terminationswhich are staggered; similarly, and as a mirror image, the axialtermination of the armature A A are likewise staggered. The radialdistances between succesive armatures A A A as well as the axial lengthsthereof are so calculated that the magnetic field will be as close tothe ideal distribution as possible, so that the insulating material,located between the layers of the windings as well as between thearmature elements will be loaded as much as possible in a transversedirection. This result is achieved when the equipotential lines, in theplane of FIG. 2, are parallel to as large an extent as possible to thehousing wall M and to housing wall M respectively. The radial distancesbetween armatures A A A are thus s'o arranged that, due to thecapacitative distribution of voltage, each of the armatures will be atan average potential corresponding to the layers of the windings C C Cand the length of the armatures A A A will then also be equal to thelength of the corresponding layers C C C of the windings.

The metallic armatures A A .A are preferably made of magnetic materialof high permeability, with sufiicient cross-section so that the materialwil not saturate magnetically due to stray magnetic fluxes. This isparticularly important in connection with extra high voltage, high powertransformers.

The electrical field distribution can further be improved by combiningauxiliary armatures B B B with the armature elements A A A The auxiliaryelements B B B are arranged radially and axially between the outer endsof the winding layers C C C and housing M Armatures B B B are carried atthe potential of the outside turn of the next adjacent winding layer C CC by interconnections, which are not shown in the drawings.

By providing a structure in which the equipotential lines of theelectrostatic shield are essentialy parallel to the metallic parts ofthe apparatus, so that the insulation will be loaded transverselythereto, the distances between the electrical windings and the metallichousings may be reduced, without simultaneously increasing losses due tostray magnetic fields in the metallic elements which surround thewindings, and in particular Within the generally substantial verticalwall of the housing surrounding the transformer.

I claim:

1. Electrical high tension inductive apparatus having concentric layerof windings (C C C said windings having a common circle ofconcentricity;

insulating material surrounding the windings,

and a housing located outside of said insulating material, comprising aplurality of radially spaced, concentric metallic armatures (A A A equalin number to the number of winding layers, said armatures beingconcentric with said windings (C C C and being located outside of thecircle of concentricity of said windings and positioned in said housingat diameters, with respect to the center of said circle ofconcentricity, to form an electrostatic image of the windings withrespect to an imaginary cylinder (E) having its axis at the center ofthe circle of concentricity, said cylinder having a greater diameterthan said circle of concentricity.

2. Apparatus according to claim 1 wherein said metallic armaturesconsist of magnetic material having high permeability and of suflieientcross-section to operate in unsaturated state during operation of theapparatus.

3. Apparatus according to claim 1 wherein the layers of said windingsare cylindrical and have progressively different axial length.

4. Apparatus according to claim 3 wherein said metallic armaturecomprises cylindrical sheet metal elements symmetrically located withrespect to said windings and forming mirror images thereof With'thelength thereof matching the length of said layer-s of windings.

5. Apparatus according to claim 3 including metal inserts locatedtransverse to the axis of said windings at the ends thereof and formingmagnetic guides for magnetic flux from said layers of windings to saidsheet metal element.

6. Apparatus according to claim 1 wherein said metallic armatures areequal in length to their corresponding winding layers.

References Cited FOREIGN PATENTS 53,188 5/1967 Germany.

THOMAS J. KOZMA, Primary Examiner U.S. Cl. X.R. 336-84

